The goals of the proposed research are to identify the epitope specificity and affinity required for immune control of antigenically-variant bacteria during acute and persistent infection and to determine whether this immune response is responsible for restricting pathogen genetic diversity in the mammalian reservoir host. This research addresses a fundamental gap in knowledge regarding control of bacterial pathogens in which the immune response is directed against both conserved and variable epitopes of outer membrane proteins. Tick-transmitted pathogens in the Family Anaplasmataceae (Order: Rickettsiales) cause acute febrile illness in animals and humans. During acute infection, cell-associated bacteremia reaches high, microscopically detectable levels and results in systemic disease. Importantly, ticks that feed on the mammalian host during this period of high-level bacteremia efficiently acquire the pathogen. Resolution of the acute high-level bacteremia requires CD4+ T lymphocytes and is associated with secretion of IFN-gamma and induction of neutralizing antibodies. This response does not completely clear the pathogen but the consequent persistent infection is controlled at low levels and the efficiency with which feeding ticks acquire the pathogen drops markedly. While studies have shown that persistence of Anaplasma spp. reflects emergence of organisms expressing structural and antigenic variants of the immunodominant outer membrane protein MSP2, how the immune response effectively controls the pathogen to low levels in face of this variation is unknown. In part 1 of the project, the epitope specificity and affinity of MSP2-specific CD4+ T cells associated with control of acute high-level bacteremia will be determined and whether induction of these responses prevents high-level bacteremia will be tested. In part 2, the epidemiological consequences of the gene conversion mechanism used to generate MSP2 variants and the effect of differential selection in the mammalian host versus tick vector will be examined. The generation of numerous complex MSP2 variants during infection of the mammalian host and the resulting immune responses are proposed to prevent tick-transmitted superinfection and thus restrict pathogen genotypic diversity. This hypothesis will be addressed using comprehensive identification of the variant population in the reservoir host and tick vector and testing whether immune responses against a broad array of variants prevents superinfection.